SURFACE-DIFFUSION IN THE 2D-PHASE TRANSITION OF CO2 ADSORBED ON NACL(100)

CO2 on NaCl(100) at coverages below one monolayer (ML) adsorbs as a 2D -lattice gas in equilibrium with an ordered 2D-condensed phase with (2 x1) structure. Diffusion of isolated CO2-molecules in the supersaturat ed 2D-lattice gas phase on NaCl(100) was studied by means of time-reso lved, high-resolution polarization Fourier-transform infrared spectros copy. The process was monitored by recording the time evolution of the v(3) vibrational spectra of adsorbed CO2 as the initially randomly di stributed molecules diffuse to thermodynamically favored step and clus ter sites. Performing the measurements at different substrate temperat ures and applying a simple diffusion model yields the barrier for the migration process of E(diff) = (3.6 +/- 0.9) kJ/mol in good agreement to results of potential energy calculations. The experimental as well as the theoretical results will be discussed within the context of the rmodynamic data obtained at higher temperatures (45 less than or equal to T/K less than or equal to 100), especially their importance for mo delling the 2D-phase transition by simulating adsorption isotherms.